Steel stair stringer/space frame?

[shacked]

I am working on a 3 story residential custom home and the Architect wants 2 sets of steel stair stringer (space frames) for the interior staircase. See attached pdf.

I do not have experience modeling something like this in a f.e.m. program but I have provided calculations for a 2d frame(steel stair stringer and landing beam)simply supported on each end.

Question is: Assuming that each joint is fixed(good welder) and x-rayed could I use the total stringer and landing lengths to determine my section sizes based on the loading? Would this produce an accurate design versus f.e.m?

I realize that there would be some torsion placed on the stringers but mostly this will be taken out at the floor connections.

F.Y.I. I am a sole proprietor and I can't justify the cost of a 3d f.e.m. program as I rarely do projects that need something like this.

Thanks

 

[r13]

If there is no edge support, the success is based on the strength of the stringers, and the floors to support the cantilever flights and potential twisting. It can be done without 3D program, but can be tedious and tricky. Yes, as usual, use the projected length for design.

 

[SteelPE]

Are you the EOR? I am not sure I would even attempt this without a 3D FEM program. Even then I would only do it kicking and screaming. I have been asked to look at something similar and the past and refused. The client relented and agreed to provide support to the landings.

Would it be possible to pass this off to the fabricator as a delegated design portion? I know you might not want to do that, but if you don't have the tools then what are you going to do.

I remember I did an L shaped steel stair 15 years ago w/o a landing. Took many 3d FEM models to narrow down what I wanted to do, and then I ended up providing much more strength than what was required. The stair was built and I still do work for the owner so I suppose that's a good sign. In my instance we used huge tube stringers and I distinct remember making a horizontal truss under the treads which made an huge difference in the design.

 

[shacked]

Thanks retired, that is pretty much what I thought.

Steel...here is the story. I am the EOR and the plans are already permitted. I told the Architect that i can't do this portion of the design because I do not have the required software, so approved as a deferred submital.

Last meeting that I had with the Architect and contractor, the contractor asked the Architect about the stair design and the Architect said, "I did this before on a previous project so I will just give the engineer the shop drawings from that project to review and ok."

Me, "NO!"

Just looking for some assurance that I am not overreacting to this design.

Thanks

 

[SteelPE]

I would say that it's not a standard design. If you are working for the architect I would explain this to them. Hopefully they can see your argument. Unfortunately, I don't know of any quick ways to handle the project. The engineering costs associated with the stair design will not be small.

Also, the loads on the floor headers is going to be tremendous. Hopefully you have covered this in your design

 

[human909]

As an alternative what will make you comfortable in designing such this? Maybe approach it from this direction.

"In my instance we used huge tube stringers and I distinct remember making a horizontal truss under the treads which made an huge difference in the design."

 

[Retrograde]

Architect said, "I did this before on a previous project so I will just give the engineer the shop drawings from that project to review and ok."

Any chance you could go look at this previous project in person. Walk up and down the flights and see how bouncy it is (if at all). Might give you some confidence in the design.

 

[sticksandtriangles]

Even if you could get the stair stringers to work, I would not want to attach to the wooden beam (?) at all due to the difficulty of connection and overall flexibility this might produce.

I think accurately capturing the supporting member stiffness is crucial to getting this stair to feel sturdy and not bouncy.

This post covers the same type of stair it looks like your section is showing:

https://www.eng-tips.com/viewthread.cfm?qid=466727

I ended up modeling in my steel beam supports for the stair and had to beef them up to get the vibration of the stair under control.




S&T